Plasma accelerator of short length with closed electron drift

ABSTRACT

The system (31 to 33, 34 to 38) for generating a magnetic field in the main channel of the plasma accelerator are adapted to produce, in the main channel (24) an essentially radial magnetic field at the downstream end (225) of the channel (24), its induction being maximum at this point. The magnetic field has a minimum induction in the transition area in the vicinity of the anode (25), the absolute induction value of the field increasing again upstream of the anode (25), in the region of the buffer chamber (23) in order to produce a magnetic mirror effect. The magnetic field lines include, between the anode (25) and the downstream end (225) of the channel (24), a concavity oriented downstream, causing focusing of ions, the maximum ionisation density area being located downstream of the anode (25). The magnetic field sources comprise several distinct magnetic field sources (31 to 33) and inner (35) and outer (34) radial, plane pole pieces (34, 35) disposed at the outlet face on either side of the main channel (24) and linked to one another by a central core (38), a yoke (36) and a peripheral magnetic circuit (37) axially disposed outside of the main channel (24). The yoke (36) consists of radial elements located in the immediate vicinity of the anode (25) and passing through the annular buffer chamber (23), thereby creating spaces (13) for communication between the annular buffer chamber (23) and the main channel (24).

FIELD OF THE INVENTION

The present invention relates to plasma accelerators applied inparticular to space propulsion and more particularly it relates toplasma accelerators of the closed electron drift type also referred toas stationary plasma accelerators or, in the USA, as "Hall currentaccelerators".

PRIOR ART

Electrical accelerators are intended essentially for space propulsionapplications. As sources of ions or of plasma, they are also used forterrestrial applications, in particular for ion machining. Because oftheir high specific impulse (1500 s to 6000 s) they enable considerablemass savings to be achieved on satellites compared with acceleratorsthat make use of chemical propulsion.

One of the typical applications of accelerators of this type isnorth-south control of geostationary satellites, where a mass saving of10% to 15% can be achieved. They may also be used for drag compensationin low orbit, for maintaining a heliosynchronous orbit, and for primaryinterplanetary propulsion.

Ion thrusters can be divided into several categories.

A first type of ion thruster, also known as a Kaufman accelerator, isthus constituted by an accelerator in which ionization is performed bybombardment. Examples of thrusters of that type are described inparticular in Documents EP-A-0 132 065, WO 89/05404 and EP-A-0 468 706.

In an accelerator making use of ionization by bombardment, atoms ofpropellant gas are injected under low pressure into a discharge chamberwhere they are bombarded by electrons emitted by a hollow cathode andcollected by an anode. The ionization process is increased by thepresence of a magnetic field. A certain number of atom-electroncollisions cause a plasma to be created whose ions are attracted by theacceleration electrodes (outlet grids) themselves at a negativepotential relative to the potential of the plasma. The electrodesconcentrate and accelerate the ions which leave the thruster in the formof widely spreading radiation. The ion radiation is then neutralized bya flow of electrons emitted from an external hollow cathode, referred toas a "neutralizer".

The specific impulse (Isp) obtained from a thruster of that type can beof the order of 3000 seconds or more.

The power requirement is of the order of 30 W per mN of thrust.

Other types of ionization accelerator are constituted by acceleratorsusing radiofrequency ionization, accelerators using ionization bycontact, or field emission accelerators.

Those various ionization accelerators, including accelerators whereionization is obtained by bombardment, have in common the fact that thefunctions of ionization and of accelerating the ions are clearlyseparate.

They also have in common the fact of presenting a current density in theion optics which is limited by the space charge phenomenon, with densitybeing limited in practice to 2 mA/cm² to 3 mA/cm² in accelerators whereionization is obtained by bombardment, thus presenting thrust per unitarea that is quite low.

In addition, such accelerators, and bombardment accelerators inparticular, require a certain number of electrical feeds (between 4 and10), thereby leading to the implementation of rather complex electroniccircuits for conversion and control.

A 1974 article by L. H. ARTSIMOVITCH et al. on the stationary plasmaengine (SPD) development program and on tests performed using the"METEOR" satellite, discloses accelerators of the closed electron drifttype, also known as stationary plasma accelerators which differ from theother categories of accelerators by the fact that ionization andacceleration are not separate and the acceleration zone includes thesame number of ions and of electrons, thereby making it possible toeliminate any space charge phenomenon.

A closed electron drift accelerator as proposed in the above article byL. H. ARTSIMOVITCH et al. is described below with reference to FIG. 2.

An annular channel 1 defined by a part 2 made of insulating material isplaced in an electromagnet comprising outer and inner annular polepieces 3 and 4 respectively placed outside and inside the part 2 ofinsulating material, a magnetic yoke 12 disposed upstream from theaccelerator, and electromagnetic coils 11 that extend over the entirelength of the channel 1 and that are connected in series around magneticcores 10 connecting the outer pole piece 3 to the yoke 12. A groundedhollow cathode 7 is coupled to a xenon feed device to form a cloud ofplasma in front of the downstream outlet of the channel 1. An annularanode 5 connected to the positive pole of an electric power supply, e.g.at 300 volts, is disposed in the closed upstream portion of the annularchannel 1. A xenon injection tube 6 co-operating with a thermal andelectrical insulator 8 opens out into an annular distribution channel 9disposed in the immediate vicinity of the annular anode 5.

Ionization and neutralization electrons come from the hollow cathode 7.The ionization electrons are attracted in the insulating annular channel1 by the electric field that extends between the anode 5 and the cloudof plasma coming from the cathode 7.

Under the effect of the electric field E and of the magnetic field Bcreated by the coils 11, the ionization electrons follow an azimuthdrift trajectory as required for maintaining the electric field in thechannel.

The ionization electrons then drift around closed trajectories insidethe insulating channel, whence the name of the accelerator.

The drift motion of the electrons considerably increases the probabilityof collisions between the electrons and neutral atoms, where collisionis the phenomenon that produces the ions (in this case of xenon).

The specific impulse obtained by conventional ion accelerators withclosed electron drift operating on xenon is of the order of 1000 secondsto 2500 seconds.

In conventional ion accelerators with closed electron drift, theionization zone is not organized, and as a result they operate well onlywith xenon, the jet is divergent (beam spread over an angle of ±20°),and efficiency is limited to about 50%.

In addition, the divergence of the jet causes the wall of the insulatingchannel to wear, which channel is normally made of a mixture of aluminaand boron nitride.

The lifetime of such an engine is about 3000 h.

It has been further proposed, in particular in an article entitled "Opensingle-lens Hall-current accelerator" by V. N. Dem'Yanenko, L. P. Zudkovand A. I. Morozov, published in August 1976 in the journal "SovietPhysics Technical Physics", Vol. 21 No. 8 pp. 987-988 that the twofunctions of the anode be separated by using on the one hand acylindrical anode and, on the other hand, an annular gas distributor.Such an embodiment enables the ionizable gas flow to be uniformlydistributed near the anode. The anode and the annular gas distributorare separated by a buffer chamber to enable a homogenization. However,the plasma accelerator disclosed in the above-mentioned article operatesin a pulsed mode with a high discharging voltage and is generally notwell suited for space propulsion applications.

Object and brief summary of the invention

An object of the invention is to remedy the drawbacks of known plasmaaccelerators, and more particularly to modify plasma accelerators withclosed electron drift so as to improve their technical characteristics,and in particular to enable the ionization zone to be better organizedbut without thereby creating a space charge as happens in ionsaccelerators using bombardment, for example.

The invention also seeks to reduce the divergence of the jet and toincrease the density of the ion jet, its electrical efficiency, specificimpulse, and lifetime.

Another objet of the invention is to reduce the mass and the size of theengine.

These objects are achieved by a plasma accelerator of short length withclosed electron drift, the accelerator comprising a main annular channelfor ionization and acceleration delimited by parts of insulatingmaterial and open at its downstream end, at least one hollow cathodedisposed outside the main annular channel adjacent to the downstreamportion thereof, an annular anode concentric with the main annularchannel and disposed at a distance from the open downstream end thereof,first and second means for feeding ionizable gas and respectivelyassociated with the hollow cathode and with the annular anode, magneticmeans for creating a magnetic field in the main annular channel and anannular buffer chamber whose size in the radial direction is at leastequal to that of the main annular channel and which extends upstreamtherefrom beyond the zone in which the annular anode is placed, thesecond means for feeding an ionizable gas opening out in the annularbuffer chamber upstream from the anode into a zone that is distinct fromthe zone including the anode, characterized in that the means forcreating a magnetic field in the main channel are adapted to produce amagnetic field in said main channel that is essentially radial at thedownstream end of the channel and has a maximum induction at this level,this magnetic field having a minimum induction in the transition zonesituated in the vicinity of the anode, the absolute value of theinduction of this magnetic field increasing again upstream from theanode, at the level of the buffer chamber to produce a magnetic mirroreffect, the magnetic field lines having, between the anode and thedownstream end of the channel, a concavity which is orientated downwardsand produces a focussing of the ions, a region located downstream fromthe anode having a maximum ionisation density, in that the means forcreating a magnetic field comprise a plurality of distinct magneticfield creation means and inner and outer plane radial pole piecesdisposed level with the outlet face on either side of the main channeland interconnected by a central core, a yoke, and a peripheral magneticcircuit disposed axially outside the main channel, the yoke being madeup of radial elements situated in the immediate vicinity of the anodeand penetrating into the annular buffer chamber, communication spacesbetween the annular buffer chamber and the main channel being leftbetween the radial elements.

Advantageously, the dimension of the buffer chamber in the radialdirection is comprised between one and twice the radial dimension of themain channel.

More particularly, the distinct magnetic field creation means comprisefirst means disposed around and outside the main channel in the vicinityof the downstream end thereof, second means disposed around the centralcore in a zone facing the anode and extending in part over the bufferchamber for the creation of the magnetic mirror effect, and third meansdisposed around the central core between the second means and thedownstream end of the main channel.

In one possible embodiment, the first, second, and third magnetic fieldcreation means are constituted by induction coils.

In particular because of the physical separation of the anode and theionizable gas manifold, because of the existence of a buffer chamber,and because a magnetic field is established having a particular profile,the plasma accelerator of the invention presents the following set ofadvantages:

a) ionization is more effective, giving rise to greater efficiency;

b) it is easy to ionize various thrust gases such as xenon, argon, etc.because of an improvement in the ionization process;

c) electrostatic equipotentials are obtained that reduce the divergenceof the beam, whence:

c1) easier integration in a satellite; and

c2) reduced wear of the acceleration channel.

More specifically, due to the provision of a particular magnetic fieldprofile within the acceleration channel upstream from the anode and inthe midst of the buffer chamber:

the homogeneity of the plasma is improved and the distorsion ofelectrostatic equipotential lines is thus reduced in the accelerationzone, which contributes to limit the ion losses on the walls and toincrease the beam focusing,

the region in which the ions are produced is better localized, whichcontributes to reduce the ion energy scattering, and

an immaterial plasma confinement is achieved upstream from the anodethrough a magnetic mirror effect.

The transition between the minimum value of the magnetic field in thevicinity of the anode and the maximum value at the output of theacceleration channel (about 300 Oe) always guarantees that a zone willbe obtained where the ionization probability is raised to a maximum.

The geometry of the buffer chamber enables an extension of the plasmaupstream from the anode and a retention in place of the plasma due tothe magnetic mirror effect.

The fact that, according to the invention, the connecting yoke betweenthe central core and the peripheral magnetic circuit is situated in theimmediate vicinity of the anode and penetrates into the annular bufferchamber, makes it possible to reduce the length and thus the mass of theentire magnetic circuit, thereby giving rise to an accelerator whosemass and dimensions are considerably smaller than those of embodimentsin which the connection yoke between the central core and the peripheralmagnetic circuit is situated upstream from the buffer chamber.

The connection yoke that passes through the buffer chamber while leavingcommunication spaces with the main channel may be implemented in variousdifferent ways.

Thus, the yoke may comprise radial elements constituted by cylindricalmagnetic bars passing through the annular chamber.

In which case, the magnetic bars may be constituted by metal bars thatare electrically insulated by two-part sheaths which parts arerespectively secured to the walls of the main channel and to the wallsof the buffer chamber.

In one particular embodiment, the magnetic bars are interconnected attheir peripherally outer ends by a continuous magnetic ring constitutinga structural part for fixing the accelerator to the structure of asatellite.

The magnetic bars may also be constituted by metal bars that areelectrically insulated from ground by ferrite parts respectivelyconstituting said central core and said peripheral magnetic circuitdisposed axially outside the main channel, the magnetic bars beingcapable of being biased to the same potential as the anode.

In another possible embodiment, the magnetic bars are constituted by aninsulating ferrite material enabling them to be directly implanted inthe buffer chamber.

The peripheral magnetic circuit may comprise a set of link bars betweenthe radially outer pole piece and the yoke, or else it may beconstituted by a shell.

The yoke may comprise bars extending radially in a plane substantiallyperpendicular to the axis of the buffer chamber and of the main channel.

However, in another possible embodiment, the yoke comprises barsextending radially along the generator lines of a truncated cone whosesmaller section end is connected to the central core, its larger sectionend being connected to the peripheral magnetic circuit, and its axiscoinciding substantially with the axis of the buffer chamber and of themain channel.

In yet another particular embodiment, the yoke comprises a frustoconicalferrite part whose smaller section end is connected to the central coreand whose larger section end is connected to a shell constituting theperipheral magnetic circuit, channels formed axially through saidfrustoconical part constituting said spaces for communication betweenthe annual buffer chamber and the main channel.

The invention further relates to a plasma accelerator, in which thebuffer chamber comprises a plurality of alveoles which open out into theacceleration channel in the vicinity of the anode, are distributedaround the axis of the accelerator and are delimited by partitions whichare parallel to the axis of the accelerator and define, between adjacentalveoles, passages for cylindrical magnetic bars which constitute theyoke without penetrating into the alveolate buffer chamber.

Such a buffer chamber may be made in one piece.

According to a possible embodiment, the second means for feeding anionizable gas open out in the annular buffer chamber upstream from theanode through an annular manifold.

In the case of an alveolate buffer chamber, the annular manifold isassociated with sonic throats opening out in the different alveoles ofthe alveolate buffer chamber.

According to another possible embodiment, the second means for feedingan ionizable gas open out in the annular buffer chamber upstream fromthe anode through a single sonic throat which is mounted tangentiallyalong the largest diameter of the buffer chamber to create a vortex.

When the mean diameter of the accelerator is large with respect to thechannel width, according to a particular embodiment, the hollow cathodeis located along the axis of the accelerator within the central tubularcore and is thermally insulated from this central core through asuperinsulating screen.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention appear from the followingdescription of particular embodiments, given as non-limiting examplesand with reference to the accompanying drawings, in which:

FIG. 1 is an elevation view and an axial half-section view of oneexample of a plasma accelerator with closed electron drift according tothe present invention;

FIG. 2 is an axial section view showing an example of a prior art plasmaaccelerator with closed electron drift;

FIG. 3 is an exploded perspective view of a fraction of the componentparts of a plasma accelerator of the invention showing a yoke havingmetal bars that are electrically isolated by two-part sheaths;

FIG. 3a shows a detail of how an insulated bar is implemented in theembodiment of FIG. 3;

FIG. 4 is an axial half-section view of a plasma accelerator of theinvention, similar to that of FIG. 1, but having different link means tothe support plate;

FIG. 5 is an axial section through a variant embodiment of a plasmaaccelerator of the invention with a yoke having link bars made offerrite;

FIG. 6 is an axial section through a variant embodiment of a plasmaaccelerator of the invention with link bars made of metal and withportions of its magnetic circuit made of ferrite;

FIG. 7 is an axial section through a particular embodiment of a plasmaaccelerator of the invention in which the link yoke is constituted bybars disposed in a cone; and

FIG. 8 is an axial section through a particular embodiment of a plasmaaccelerator of the invention in which the link yoke is constituted by aconical shell pierced by axial link channels.

FIG. 9 is an axial section through a particular embodiment of a plasmaaccelerator of the invention comprising a buffer chamber whichconstitutes a cylindrical extension of the acceleration channel withoutany increase of the outer diameter,

FIG. 10 is an axial section through a particular embodiment of a plasmaaccelerator of the invention comprising a buffer chamber which has areduced length and is associated with a tangential gas injector,

FIG. 11 is a half-section view along plane XI--XI of FIG. 10,

FIG. 12 is an axial section through a particular embodiment of a plasmaaccelerator of the invention, comprising a buffer chamber divided into aplurality of alveoles between which are located magnetic bars,

FIG. 13 is a perspective exploded view showing a buffer chamber made inone piece and a set of magnetic bars which may be incorporated in theplasma accelerator of FIG. 12, and

FIG. 14 is an axial reaction through a particular embodiment of a plasmaaccelerator of the invention which has a mean diameter larger than thewidth of the acceleration channel, and comprises a hollow cathode whichis located within a central polar piece shaped in a hollow tube.

Detailed description of particular embodiments

FIG. 1 shows an example of a plasma accelerator 20 with closed electrondrift according to the invention comprising a set of parts 22 made ofinsulating material delimiting an annular channel 21 formed at itsupstream end by a first portion constituted by a buffer chamber 23 andat its downstream end by a second portion constituted by an accelerationchannel 24.

The dimension of the annular chamber 23 in the radial direction ispreferably between once and twice the dimension of the annularacceleration channel 24 in the radial direction. In the axial direction,the buffer chamber 23 may be a little shorter than the accelerationchannel 24 and advantageously its length is between about one andone-and-a-half times the dimension d in the radial direction of theacceleration channel 24.

An anode 25 is connected by an electrical line 43 to a DC voltage source44 (which may be at about 200 V to 300 V) and is disposed on theinsulating parts 22 delimiting the annular channel 21 in a zone situatedimmediately downstream from the buffer chamber 23, at the inlet to theacceleration channel 24. The line 43 for powering the anode 25 isdisposed in an insulating tube 45 which passes through parts 223 and 224of insulating material that delimit the buffer chamber 23.

A tube 26 for feeding an ionizable gas such as xenon also passes throughthe end wall 223 of the buffer chamber 23, opening out into an annulargas manifold 27 placed at the end of the buffer chamber

The channel 21 delimited by the set of insulating parts 22 is placed ina magnetic circuit essentially constituted by three coils 31, 32, and33, and by pole pieces 34 and 35.

Outer and inner plane pole pieces 34 and 35 are placed in the outletplane of the accelerator outside the acceleration channel 24 and set upmagnetic field lines that are substantially parallel to the outlet plane59 of the accelerator 20 in the downstream open portion of theacceleration channel 24.

The magnetic circuit constituted by the pole pieces 34 and 35 is closedby an axial central core 38 and by link bars 37 disposed at theperiphery of the accelerator in an essentially cylindricalconfiguration, the central core 38 of ferromagnetic material and thelink bars 37 of ferromagnetic material coming into contact with a rearlink yoke 36 of ferromagnetic material. The yoke 36 is constituted byelements that are essentially radial and which are situated in theimmediate vicinity of the anode 25, penetrating into the buffer chamber23 and leaving between them communication spaces 136 between the bufferchamber 23 and the annular channel 24.

An antipollution or antiradiation screen 39 may also be disposed betweenthe insulating parts 22 and the link bars 37. The link bars 37 and thescreen 39 may nevertheless be replaced by a cylindrical or acylindroconical shell which acts simultaneously as an antipollutionscreen and to close the magnetic circuit.

The electrons required for operation of the accelerator are provided bya hollow cathode 40 which may be conventional in design. The cathode 40which is electrically connected by a line 42 to the negative pole of thevoltage source 44 includes a circuit 41 for feeding it with an ionizablegas such as xenon, and it is located downstream from the outlet zone ofthe acceleration channel 24.

The hollow cathode 40 provides a plasma 29 substantially at thereference potential, with electrons being extracted therefrom andtravelling towards the anode 25 under the effect of the electrostaticfield E due to the potential difference between the anode 25 and thecathode 40.

These electrons have an azimuth drift trajectory in the accelerationchannel 24 under the effect of the electric field E and of the magneticfield B.

Typically, the field at the outlet from the channel 24 is 150 Oe to 200Oe.

The primary electrons are accelerated by the electrostatic field E, andthey thus strike the wall of the insulator 22 which provides secondaryelectrons of lower energy.

The electrons come into collision with the neutral atoms of xenon fromthe buffer chamber 23.

The xenon ions formed in this way are accelerated by the electrostaticfield E in the acceleration channel 24.

There is no space charge in the acceleration channel 24 because of thepresence of the electrons.

The ion beam is neutralized by a fraction of the electrons coming fromthe hollow cathode 40.

The control obtained over the gradient of the radial magnetic fieldbecause of the disposition of the coils 31 to 33 and of the pole pieces34 and 35 makes it possible to separate the function of accelerating theions from the ionization function obtained in a zone close to the anode25. This ionization zone may extend partially into the buffer chamber23.

An important feature of the accelerator of the invention lies in theexistence of a buffer chamber 23 which makes it possible to optimize theionization zone.

In conventional accelerators with closed electron drift0, a considerablefraction of ionization takes place in the middle portion. Some of theions strike the walls and this is a cause of rapid wall wear, therebyreducing the lifetime of the thruster. The buffer chamber 23 helpsreduce the radial gradient of plasma concentration and also helps reducethe cooling of electrons at the inlet to the acceleration channel 24,thereby reducing the divergence of the ion beam against the walls, andthus avoiding the loss of ions by collision with the walls, therebyhaving the effect of increasing efficiency and of reducing thedivergence of the beam at the outlet from the accelerator.

Another important feature of the motor of the invention lies in thepresence of the three coils 31 to 33 which may be of differentdimensions and which enable the magnetic field to be optimized by virtueof their specific positioning.

Thus, a first coil 31 is disposed around and outside the main channel 24in the vicinity of the downstream end 225 thereof. A second coil 32 isdisposed around the central core 38 in a zone facing the anode 25 andcapable of extending partially over the buffer chamber 23 to enable thecreation of a magnetic mirror effect (FIGS. 7 and 8). A third coil 33 isdisposed around the central core 38 between the second coil 32 and thedownstream end 225 of the main acceleration channel 24. The coils 31,32, and 33 may be of different sizes. The consequence of having threewell-differentiated coils 31, 32, and 33 is to create field lines thatare better directed, thus making it possible to obtain a jet that ischanneled better, and in particular that is more parallel, than inconventional accelerators.

The created magnetic field is essentially radial at the end 225 of themain acceleration channel 24 and has a magnetic induction which is at amaximum at this level. The magnetic field has a minimum value, which maybe equal to zero, in the vicinity of the anode. The absolute value ofthe magnetic field increases again upstream from the anode 25 inparticular within the buffer chamber 23. This configuration of themagnetic field creates a magnetic mirror effect which prevents theplasma from propagating into the buffer chamber 23.

In a variant embodiment, the magnetic field coils 31 to 33 may bereplaced, at least in part, by permanent magnets having a Curie pointhigher than the operating temperature of the accelerator.

The annular coil 31 could also be replaced by a set of individual coilsdisposed around the various different link bars 37 constituting theperipheral magnetic circuit.

The material of the magnetic circuit constituted by the pole pieces 34and 35, the central core 38, the bars 37, and the yoke 36 may be softiron, ultrapure iron, or an iron-chromium alloy having high magneticpermeability.

By way of example, the pole pieces 34 and 35 may extend about 20millimeters in the axial direction.

The number of ampere-turns in each coil 31, 32, 33 and the ratio of thelength to the diameter of each coil are determined so as to produce amagnetic field in the acceleration channel that is essentially radial,with the maximum thereof being situated in the outlet plane 59 of theaccelerator, with its field lines close to the outlet 225 beingessentially parallel to the outlet face 59, and with its field lines inthe vicinity of the anode 25 being essentially disposed so as to enhanceionization of the thrust gas in this region.

Examples of ion thrusters of the invention combining the presence of abuffer chamber 23 and of a set of distinct coils 31, 32, and 33 haveachieved electrical efficiencies of about 50% to 70%, thus giving anaverage improvement of about 10% to about 25% over previously knownsystems.

In addition, the jet obtained at the outlet from embodiments of theinvention is practically cylindrical, with very little divergence of thejet of ions, the divergence being about ±9°. Thus, using an accelerationchannel having an outside diameter of 80 mm, 90% of the energy remainsconcentrated in the diameter of the acceleration channel at a distanceof 80 mm outside the accelerator relative to its outlet plane 59.

In general, the accelerator of the invention enables a higher thrustdensity to be obtained (e.g. about 1 mN/cm² to 2 mN/cm² thrust densityper unit area), thus making it possible to have an accelerator that issmaller and lighter for given thrust, with excellent efficiency beingobtained.

As for lifetime, known accelerators have a lifetime of about 3000 h.

In contrast, a plasma accelerator of the present invention makes itpossible to obtain a lifetime of at least 5000 hours to 6000 hoursbecause of the reduced erosion of the channel 24 due to the morecylindrical ionized jet.

The plasma accelerator of the invention may be implemented in numerousdifferent ways.

In the example shown in FIG. 1, a magnetic circuit is shown thatcomprises an outer pole piece 34, an inner pole piece 35, a magneticcore 38, a link yoke 36, and 5 axial ferromagnetic bars 37 which extendto an outer ring 36A that forms a portion of the link yoke 36 and thatacts as a structural component suitable for fixing directly to theassembly plate for the accelerator on a satellite, thereby creating afixing zone that is very close to the center of gravity of theaccelerator, thus improving vibration performance, or else, and as shownin FIG. 1, the accelerator is connected to the assembly plate by anon-magnetic cylindrical shell 69 which thus constitutes an assemblyinterface.

The link yoke between the central magnetic core 38 and the axialferromagnetic bars 37 is constituted by radial bars 36 of ferromagneticmaterial passing through the buffer chamber 23 just upstream of the mainchannel 24 and the anode 25, leaving large communication spaces 136therebetween for communication between the buffer chamber 23 and themain channel 24, as shown more clearly in FIG. 3.

The number of bars 36 may, for example, lie in the range three to nine.The outer ring 36A is in the form of a washer and may be formedintegrally with the bars 36.

In the embodiment of FIGS. 1, 3, and 3A, the bars 36 are shown as beingelectrically insulated by insulating sheaths 141 and 142. The sheaths141 and 142 are advantageously made in two portions 141 and 142 securedrespectively to the walls 22 of the main channel 24 and to the walls 224of the buffer chamber 23. More particularly, in the embodiment shown inFIGS. 3 and 3A, the bars 36 are semicylindrical in section, eachhalf-sheath 141 having a section that fits over the semicylindricalshape of a bar 36, while each half-sheath 142 is plane in shape andoverlies the plane face of a bar 36.

FIG. 4 is an axial half-section and perspective view showing a variantembodiment in which the bars 36 constitute radial arms that are notinterconnected by a ring 36A at their outer ends. The various axial bars37 are then connected directly to the outer ends of the radial bars 36.Each bar 36 is also connected by a spacer 146 to the baseplate 145 formounting on a satellite. The central core 38 is itself held by anextension 147 of the baseplate 145.

For reasons of clarity, FIG. 3, FIG. 4, and FIGS. 5 to 8 do not showvarious items shown in FIG. 1, e.g. the electrical feed means for theanode 25.

In the embodiment of FIG. 5, the axial bars 37 are replaced by an outershell 37a of ferromagnetic material. The radial bars 36 are themselvesmade of electrically insulating soft ferrite. The bars 36 therefore haveno need to be surrounded by insulating sheaths 141 and 142 as in theembodiments of FIGS. 1, 3, and 4. When the bars 36 are made of softferrite, the electrostatic field is not disturbed in the vicinity of thebars 36.

Sealing may be obtained between the bars 36 and the insulating ceramicwalls 22 of the main channel 24 by using a glass sealant or a cement,providing that the ceramic and the ferrite are selected so as to havecoefficients of expansion that are similar.

By way of example, the particular configuration of FIG. 5 has sevenradial cylindrical bars 36 made of ferrite closing the magnetic circuitbetween the outer shell 37a and the central core 38.

In the embodiment of FIG. 6, the link bars 36 are made of aferromagnetic metal, but they are not surrounded by insulating sheaths.In contrast, the central core 38 and the parts 37b constituting theaxially outer portion of the magnetic circuit (in the form of bars or inthe form of a shell) are made of electrically insulating ferrite.

Under such circumstances, the metal bars 36 may be biased to the samepotential as the anode and may act as the anode 25 or as an additionalanode.

FIG. 7 shows an embodiment in which the radial link bars 36 are nolonger disposed in a plane perpendicular to the axis of the accelerator,but are disposed along the generator lines of a cone whose base isdirected towards the downstream end of the accelerator. The base of thecone is thus formed by a shell 37a constituting the axially outerportion of the magnetic circuit while the apex of the cone, or the smallsection of the truncated cone are connected to the central core 38through the buffer chamber 23. This embodiment makes it possible toimplement a long coil 32 in the vicinity of the junction between thebuffer chamber 23 and the main channel 24.

FIG. 8 shows an embodiment in which the link yoke 36 is not made up ofdistinct bars but is constituted by a conical piece of ferrite whoselarge base is directed towards the downstream end of the accelerator andis connected to the cylindrical shell 37a constituting the axially outerportion of the magnetic circuit, while its apex is connected to thecentral core 38, the conical part 36 passing through the buffer chamber23 upstream from the anode 25. The buffer chamber 23 is thus subdividedinto two cavities which communicate via channels 136 pierced axiallythrough the conical part 36. The number of channels 136 or the sectionthereof is large enough to present negligible impedance to the flow ofgas.

As in the embodiment of FIG. 7, implementing the link yoke 36 in theform of a cone that passes through the buffer chamber 23 upstream fromthe anode 25 makes it possible to dispose a relatively long coil 32 inthe vicinity of the junction between the buffer chamber 23 and the mainchannel 24.

FIG. 9 shows a plasma accelerator according to the invention, in whichthe buffer chamber 23 constitutes a cylindrical extension of theacceleration channel 24. In such a case, the transversal dimension ofthe buffer chamber 23, and the outer diameter of the latter are the sameas for the acceleration channel 24.

The set of pieces 222, 223, 224 defining the annular channel 21 whichcomprises in a sequence the buffer chamber 23 and the accelerationchannel 24 shows on the outer face of its wall 224, perpendicularly tothe accelerator axis, a flange 323 for its mounting on an interfaceflange 145 against which abuts the outer shell 37a constituting theaxially outer portion of the magnetic circuit. The interface plane wherethe accelerator may be fixed on the supporting structure of thesatellite bears the reference 245.

The structure of the accelerator of FIG. 9 may otherwise be similar forexample to the embodiment of FIG. 5. The annular manifold 27 for feedingan ionizable gas may however preferably be located near the bottom 223of the buffer chamber 23 in the vicinity of the inner piece 222 whichboth delimits the buffer chamber 23 and the acceleration channel 24.

FIGS. 10 and 11 show a plasma accelerator according to the invention inwhich the buffer chamber 23 in the longitudinal direction has a reducedlength, which may even be slightly smaller than the transversaldimension of the acceleration channel 24.

In this case, the annular manifold 27 is replaced by a tangential gasinjector 227 which comprises a sonic throat enabling a tangential inputof gas into the buffer chamber 23 with a vortex effect which permits ahomogenization of the gas flow notwithstanding the small longitudinaldimension of the buffer chamber 23. The other parts of the acceleratorof FIGS. 10 and 11 may be constituted for example according to theembodiment of FIG. 6 and will not be described again.

FIG. 12 shows a particular embodiment of a plasma accelerator accordingto the invention, in which the buffer chamber 23, which is shown as aperspective view on FIG. 13, comprises a plurality of alveoles whichopen out into the acceleration channel 24 in the vicinity of the anode25, are distributed around the axis of the accelerator and are delimitedby partitions which are parallel to the axis of the accelerator. Thepartitions which are essentially parallel to the axis of the motordefine, between adjacent alveoles, passages 423 for magnetic bars 36which constitute the yoke. In this case, the magnetic bars 36 do notphysically penetrate into the buffer chamber 23 which may be made in onepiece and may be fabricated for example by techniques of glass or quartzblowing. The buffer chamber 23, which is to some extent moulded aroundthe bars, may be made in a mould rather than by a blowing technique. Thewalls 223 of the alveolate buffer chamber 23 are made in a materialwhich is different from the material of the cylindrical portion 22 ofthe acceleration channel 24. The junction between the downstream end ofthe walls 223 of the alveolate buffer chamber 23 and the upstream end ofthe walls 22 of the annular channel 21 bearing the anode 25 bears thereference 523.

The annular manifold 27 may be mounted in advance on the wall of thebuffer chamber 23. The annular manifold 27 is associated with sonicthroats 127 which open out in the different alveoles of the alveolatebuffer chamber 23. As can be seen on FIG. 12, the injection mayadvantageously be made towards the upstream end, the annular manifold 27being itself located downstream from the buffer chamber 23. Theinjection proper of ionizable gas is always made at a certain distanceupstream from the anode 25.

The buffer chamber 23 may comprise for example from three to ninealveoles, the magnetic bars 36 being located within the passages 423,and the number of magnetic bars being equal to the number of alveoles.

The set comprising the magnetic circuit constituted by parts 36, 38, 35and coils 32 and 33 may be introduced through the rear part of thebuffer chamber 23.

FIG. 14 shows a particular embodiment of the invention which may beapplied to a plasma accelerator whose acceleration channel 24 has a meandiameter which is important with respect to the channel width. In thiscase, the central polar piece 38 may be tubular in shape, a central freespace being reserved for inserting the hollow cathode 40 which is thenlocated along the axis of the motor. To avoid that the coils 32, 33 beoverheated by the cathode 40, a super insulating screen 140, which mayfor example be conical in shape and open at the downstream end, islocated around the cathode 40 to authorize an expansion of the beam ofthe cathode 40 only towards the free space. The cathode 40 is kept in adefinite position with respect to the tubular central polar piece 38 bymeans of a mechanical support 240.

FIGS. 12 and 14 show the interface flange 145 which is located near thelink between the bars 36 and the outer shell 37a and serves for mountingthe accelerator on a satellite.

In all of the embodiments described, the fact that the magnetic circuitdoes not go to the end of the accelerator upstream from the bufferchamber 23 makes it possible to reduce the total length and mass of theaccelerator, without impeding operation thereof.

What we claim is:
 1. A plasma accelerator of short length with closedelectron drift, comprising a main annular channel for ionization andacceleration (24) delimited by parts (22) of insulating material andopen at its downstream end (225), at least one hollow cathode (40)disposed outside the main annular channel (24) adjacent to thedownstream portion thereof, an annular anode (25) concentric with themain annular channel (24) and disposed at a distance from the opendownstream end (225) thereof, first and second means (41, 26) forfeeding ionizable gas and respectively associated with the hollowcathode (40) and with the annular anode (25), magnetic means (31 to 33,34 to 38) for creating a magnetic field in the main annular channel(24), and an annular buffer chamber (23) whose size in the radialdirection is at least equal to that of the main annular channel (24) andwhich extends upstream therefrom beyond the zone in which the annularanode (25) is placed, the second means (26) for feeding an ionizable gasopening out in the annular buffer chamber (23) upstream from the anode(25) into a zone that is distinct from the zone including the anode(25),characterized in that the means (31 to 33, 34 to 38) for creating amagnetic field in the main channel (24) are adapted to produce amagnetic field in said main channel (24) that is essentially radial atthe downstream end (225) of the channel (24) and has a maximum inductionat this level, this magnetic field having a minimum induction in thetransition zone situated in the vicinity of the anode (25), the absolutevalue of the induction of this magnetic field increasing again upstreamfrom the anode (25), at the level of the buffer chamber (23) to producea magnetic mirror effect, the magnetic field having, between the anode(25) and the downstream end (225) of the channel 24), a concavity whichis orientated downwards and produces a focussing of the ions, a regionlocated downstream from the anode (25) having a maximum ionisationdensity, in that the means for creating a magnetic field comprise aplurality of distinct magnetic field creation means (31 to 33) and innerand outer plane radial pole pieces (35, 34) disposed level with theoutlet face on either side of the main channel (24) and interconnectedby a central core (38), a yoke (36), and a peripheral magnetic circuit(37) disposed axially outside the main channel (24), the yoke (36) beingmade up of radial elements situated in the immediate vicinity of theanode (25) and penetrating into the annular buffer chamber (23),communication spaces (13) between the annular buffer chamber (23) andthe main channel (24) being left between the radial elements.
 2. Aplasma accelerator according to claim 1, characterized in that thedimension of the buffer chamber (23) in the radial direction iscomprised between once and twice the radial dimension of the mainchannel (24).
 3. A plasma accelerator according to claim 1,characterized in that the distinct magnetic field creation means (31 to33) comprise first means (31) disposed around and outside the mainchannel (24) in the vicinity of the downstream end (225) thereof, secondmeans (32) disposed around the central core (38) in a zone facing theanode (25) and extending in part over the buffer chamber (23) for thecreation of the magnetic mirror effect, and third means (33) disposedaround the central core (38) between the second means (32) and thedownstream end (225) of the main channel (24).
 4. A plasma acceleratoraccording to claim 3, characterized in that the first, second, and thirdmagnetic field creation means (31, 32, 33) are constituted by inductioncoils.
 5. A plasma accelerator according to claim 1, characterized inthat the buffer chamber (23) comprises a plurality of alveoli which openout into the acceleration channel (24) in the vicinity of the anode(25), are distributed around the axis of the accelerator and aredelimited by partitions which are parallel to the axis of theaccelerator and define, between adjacent alveoli, passages (423) forcylindrical magnetic bars which constitute the yoke (36) withoutpenetrating into the alveolate buffer chamber (23).
 6. A plasmaaccelerator according to claim 5, characterized in that the alveolatebuffer chamber (23) is made in one piece.
 7. A plasma acceleratoraccording to of claim 1, characterized in that the yoke (36) includesradial elements constituted by cylindrical magnetic bars passing throughthe annular chamber (23).
 8. A plasma accelerator according to claim 7,characterized in that the magnetic bars (36) are constituted by metalbars that are electrically insulated by two-part sheaths (141, 142)which parts are respectively secured to the walls (22) of the mainchannel (24) and to the walls (224) of the buffer chamber (23).
 9. Aplasma accelerator according to claim 7, characterized in that themagnetic bars (36) are interconnected at their peripherally outer endsby a continuous magnetic ring (36A) constituting a structural part forfixing the accelerator to the structure of a satellite.
 10. A plasmaaccelerator according to claim 7, characterized in that the magneticbars (36) are constituted by metal bars that are electrically insulatedfrom ground by ferrite parts (37b, 38b) respectively constituting saidcentral core (38) and said peripheral magnetic circuit (37) disposedaxially outside the main channel (24), the magnetic bars (36) beingcapable of being biased to the same potential as the anode (25).
 11. Aplasma accelerator according to claim 7, characterized in that themagnetic bars (36) are constituted by an insulating ferrite materialenabling them to be directly implanted in the buffer chamber (23).
 12. Aplasma accelerator according to claim 7, characterized in that theperipheral magnetic circuit (37) comprises a set of link bars betweenthe radially outer pole piece (34) and the yoke (36).
 13. A plasmaaccelerator according to claim 1, characterized in that the peripheralmagnetic circuit (37) is constituted by a shell.
 14. A plasmaaccelerator according to any one of claim 1, characterized in that theyoke (36) comprises bars extending radially in a plane substantiallyperpendicular to the axis of the buffer chamber (23) and of the mainchannel (24).
 15. A plasma accelerator according to claim 1,characterized in that the yoke (36) comprises bars extending radiallyalong the generator lines of a truncated cone whose small section end isconnected to the central core (38), its larger section end beingconnected to the peripheral magnetic circuit (37), and its axiscoinciding substantially with the axis of the buffer chamber (23) and ofthe main channel (24).
 16. A plasma accelerator according to claims 1,characterized in that the yoke (36) comprises a frustoconical ferritepart whose smaller section end is connected to the central core (38) andwhose larger section end is connected to a shell (37a) constituting theperipheral magnetic circuit (37), channels (136) formed axially throughsaid frustoconical part constituting said spaces for communicationbetween the annual buffer chamber (23) and the main channel (24).
 17. Aplasma accelerator according to claim 1, characterized in that thesecond means (26) for feeding an ionizable gas open out in the annularbuffer chamber (23) upstream from the anode (25) through an annularmanifold (27).
 18. A plasma accelerator according to claim 5 and claim17, characterized in that the annular manifold (27) is associated withsonic throats (127) opening out in the different alveoles of thealveolate buffer chamber (23).
 19. A plasma accelerator according toclaim 1, characterized in that the second means (26) for feeding anionizable gas open out in the annual buffer chamber (23) upstream fromthe anode (25) through a single sonic throat (227) which is mountedtangentially along the largest diameter of the buffer chamber to createa vortex.
 20. A plasma accelerator according to claim 1, characterizedin that the hollow cathode (40) is located along the axis of theaccelerator within the central tubular core (38) and is thermallyinsulated from this central core (38) through a superinsulating screen(140).